FIG. 1 illustrates a communication system 1 comprising several network nodes 2, 3, 4, 7, 8 providing wireless communication channels to wireless devices. The network nodes 2, 3, 4, 7, 8 may comprise enhanced Node Bs (eNodeB) or antenna points, and each network node 2, 3, 4, 7, 8 covers a certain area, cell, within which it provides the communication channels to wireless devices. A cell may be associated to one or more of an operational carrier, a radio access technology, an antenna system, a transmission power, a pilot or reference signal, etc. The cells may, but need not, be partly or completely overlapping. In FIG. 1, a first network node 2 provides a large cell within which a second and third network node 3, 4 are located. This scenario may be referred to as a macro/pico cell deployment. The first network node 2 provides the macro cell, and the second and third network nodes 3, 4 provide pico cells, the coverage of which lies entirely or partly within the macro cell. The first network node 2 may be denoted macro cell 2 and the second and third network nodes 3, 4 as pico cells 3, 4.
In the above scenario, by letting the second and third network nodes (pico cells) 3, 4 broadcast the same information as the first network node (macro cell) 2, for example system information such as primary synchronization signals/secondary synchronization signals (PSS/SSS) and broadcast control channels (BCH), several advantages can be obtained, e.g. significant energy savings on the network side. This means that a wireless device does not perceive the pico cells 3, 4 as creating cells on their own. Instead, the wireless device considers both the pico cells 3, 4 and the macro cell 2 as one cell: a soft cell. The pico cells 3, 4 may comprise radio remote units or complete base stations, e.g. eNodeBs, with rather good backhaul (indicated in the FIG. 2 at reference numeral 9). In soft cells, a node hosting one or more antennas is denoted antenna point.
The soft cell enables the use of different sets of antenna points in the uplink and the downlink. For example, and with reference to FIG. 2, some wireless devices 5, 6 may use only the macro cell 2 for some downlink (DL) channels, while using a pico cell 3, 4 and/or the macro cell for uplink (UL) channels. DL channels are illustrated in the FIG. 2 by dashed arrows going from the first network node 2 to the wireless devices 5, 6 and the UL channels are illustrated by solid arrows going from the wireless devices 5, 6 to the network nodes 2, 3, 4.
A wireless device 5, 6 wanting to access the communication network 1 and obtain services therefrom registers and a random access procedure is initiated. The random access (RA) serves as an uplink control procedure to enable the wireless device 5, 6 to access the network 1. Since an initial access attempt cannot be scheduled by the network 1, the RA procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. To include user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronization, is separated from the transmission of user data.
In traditional deployments each cell is configured with a set of random access preambles and random access resources. When joining the antenna points serving these cells into a soft cell (also denoted combined cell or shared cell) they will share system information, as described above. Then, there will be only one set of random access preambles and random access resources for the soft cell. Of course, more random access resources could be configured up to a certain limit, but it is not possible to manage two different wireless devices 5, 6 using the same random access preamble in the same random access resources. Therefore, spatial random access gains are lost, potentially making the random access channel capacity a limiting factor for soft cell operations.
FIG. 3 illustrate these problems, where two different wireless devices UE1, UE2 transmit (see arrows denoted 1) the same random access preamble in the same random access resource in a soft cell. The preambles are received by two antenna points (Antenna Point 1 and Antenna Point 2) and forwarded to a network node. The network node sends (see arrows denoted 2) a random access response over both antenna points and thereby reaches both wireless devices UE1, UE2 with an UL grant. Both wireless devices UE1, UE2 transmit (see arrows 3) using the UL grant, and these UL transmissions are received via the antenna points and forwarded to the network node. However, only the first wireless device UE1 is decoded in the network node (see arrow 3). The network node transmits (see arrow 4), via the antenna points a downlink message, aimed at the first wireless device UE1, and which the second wireless device UE2 cannot decode. Instead, the second wireless device UE2 will have to restart the random access procedure with a new preamble and resource selection.
Besides the obvious drawback for the user of the wireless device not obtaining access, and forced to perform the random access procedure again resulting in a delayed access, such insufficiency of radio resources also entails problems on a system level, e.g. in that soft cell solutions may be offered to users at the cost of lost random access spatial reuse.